The present invention relates to a method for mapping bits which are not to be transmitted, that is to say so-called DTX bits, onto a frame to be transmitted, particularly onto a compressed frame to be transmitted using the so-called compressed mode.
Mobile radio technology is subject to rapid development. At the moment, work is being carried out on the standardization of the so-called UMTS Mobile Radio Standard (“Universal Mobile Telecommunication System”) for third-generation mobile radios.
Information to be transmitted via a mobile radio channel is normally transmitted in the form of a pre-defined frame and time slot structure. A UMTS frame includes 15 time slots “slots”, with specific system information also being transmitted as well as the actual data within each frame. This system information includes, in particular, a known pilot bit sequence or training sequence, which the respective receiver can use to estimate the channel impulse response of the respective mobile radio channel, power control information in the form of one or more TPC bits (Transmit Power Control), whose contact is used to control the transmission power of the respective receiver, and format identification information in the form of so-called TFCI bits (Transport Format Combination Indicator).
On the basis of the current status of UMTS standardization, a TFCI code word is provided for each UMTS frame, including ten initially uncoded bits which are then coded using a second-order (32, 10) subcode of the Reed-Muller code, and are thus mapped onto a total of 32 bits. Of these 32 bits, bit nos. 0 and 16 are then punctured in the normal mode (in the normal mode or non-compressed mode), so that the TFCI code word now includes only 30 TFCI bits, which are then mapped or distributed uniformly with two TFCI bits in each case onto the individual time slots in the corresponding UMTS frame. These are allocated in such a way that the two most significant TFCI bits in the TFCI code word are allocated to the time slot no. 0 which is transmitted first within the UMTS frame, and the two least significant bits are allocated to the time slot no. 14, which is transmitted last within the frame. The more significant TFCI bit is then transmitted before the less significant TFCI bit within the individual time slots. The mapping or allocation of the TFCI bits in the TFCI code word onto or to the individual time slots in a frame is also referred to as mapping.
The term “puncturing” for the purposes of the present application also includes the removal or non-transmission of specific bits; in particular, the last bits.
In addition to normal transmission of information in uncompressed form, a compressed mode is also provided for data transmission. In the compressed mode, the information in the respective frame is transmitted in compressed form in order to artificially produce a transmission gap, during whose duration the absence of transmitted information can be used, for example, for intermediate-frequency measurements in order to prepare for handover processes, etc.
In the compressed mode, at least eight time slots still must be left free per frame. The 30 TFCI bits must, in consequence, be distributed between the remaining time slots in the compressed mode. In order to allow this, the time slot format of the uplink control channel DPCCH (Dedicated Physical Control Channel) and of the downlink control channel DPCCH, as well as that of the downlink data channel DPDCH (Dedicated Physical Data Channel) must be matched.
In this context, various time slot formats have been proposed for the uplink DPCCH control channel for the compressed mode. These can be summarized by the table shown in FIG. 4, in which the number NTFCI of TFCI bits transmitted per time slot and the total number D of TFCI bits transmitted per frame are in each case shown for a different number of time slots or slots transmitted per frame in the compressed mode.
Corresponding proposals for time slot formats for the downlink in the compressed mode have also been made, which can be summarized by the tables shown in FIG. 5A and FIG. 5B, where FIG. 5A relates to a spread factor of between 128 and 512 being used for the corresponding channelization codes or spread codes, while FIG. 5B relates to spread factors of between 4 and 64. Analogously to FIG. 4, these tables each show the number NTFCI of TFCI bits transmitted per time slot and the total number D of TFCI bits transmitted per frame for a different number of time slots or slots transmitted per frame in the compressed mode, with a distinction also being drawn in this case between Type A and Type B transmission.
Since it is desirable to use a standard time slot format for each frame, situations may occur (as is indicated by the individual values for D in FIG. 4 and FIGS. 5A/B) in which more TFCI points are available in each frame than are actually required for the 30 TFCI bits.
For the uplink, that is to say for transmission from a mobile part to a base station, it has thus been proposed that selected TFCI bits be repeated in the compressed mode; that is to say, that they be reiterated, in order to fill the excess TFCI points, with, in particular, those bits which are sent immediately after the transmission gap that occurs in the compressed mode being repeated at free TFCI points for this purpose, in order that the repetition is carried out as effectively as possible. The reason for this is based on the fact that the transmission power control is very uncertain immediately after the transmission gap, so that the probability of a transmission being subject to interference is highest immediately after the transmission gap, so that these bits should be repeated, if possible. The repeated bits can, in this case, be determined via the following algorithm, where ck denotes the TFCI bits, dk denotes the repeated bits, D denotes the number of TFCI points available in total in the frame, and E denotes the index or the position of that TFCI point which immediately follows the transmission gap in the compressed mode:
dD-31=cE mod 30 
dD-32=c(E-1)mod 30 
dD-33=c(E-2)mod 30 
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d0=c(E-(D-31))mod 30 
The bits are allocated to the individual time slots in the compressed frame in a descending sequence, with the TFCI bits ck being transmitted first, followed by the repeated bits dk, that is to say the bit c29 (Most Significant Bit (MSB) in the TFCI code word) being transmitted as the first bit in the TFCI code word, while d0 is transmitted as the last bit in the TFCI code word.
For the downlink, that is to say for transmission from a base station to a mobile part, it has in contrast been proposed to fill the free or excess TFCI points with so-called DTX bits (Discontinuous Transmission Bits) in the compressed mode. A DTX bit, in this case, corresponds to a bit which is not transmitted; that is to say, a bit whose energy is zero. A transmission pause with a time duration of one DTX bit is thus inserted at each of the appropriate points in the relevant time slots.
Against the background of the prior art described above, the present invention is directed toward a method for mapping bits which are not to be transmitted onto a frame which is to be sent in a compressed mode, which makes it possible to improve the transmission power and the transmission reliability without any additional complexity.